1. Technical Field
The invention relates to data storage on phase change media. More particularly, the invention relates to the repeated overwriting of the same data on phase change media.
2. Description of the Prior Art
Digital data storage systems such as the Power Drive (PD), Digital Video Disc (DVD), and Compact Disk (CD) rewritable systems, store information on a disc by exploiting the fact that certain materials or compounds can solidify in more than one phase. For example, tellurium (typically in combination with other elements) can solidify in either an amorphous phase or a crystalline phase. The two phases have different optical properties and therefore can be used to store information that can be read optically. These types of materials are typically referred to as phase change materials or phase change media.
In a typical recording scheme, a laser beam is focused onto a small spot in the layer of phase change medium on a spinning disc. If there is sufficient power in the laser beam the phase change medium softens or melts. Depending on whether the medium is cooled quickly or slowly, it solidifies in the amorphous or crystalline phase. The rate of cooling can be controlled by how quickly or slowly the laser power is turned down after melting the medium. This control of the variation of the laser power allows a sequence of amorphous or crystalline marks to be recorded on the disc. By using an appropriate modulating coding scheme, data can be recorded on the medium.
One of the key advantages of phase change recording is that data can be overwritten a large number of times on the phase change media. However, a problem occurs if the same marks are overwritten many times. In that scenario, one portion of the medium is repeatedly solidified in one state, while adjacent portions solidify in the other state. This causes a gradual flow of the phase change material from one portion of the medium to another, resulting in a degradation of the material. This in turn causes a deterioration in the integrity of the recorded data, thereby limiting the number of overwrites that are possible without risking loss of data integrity.
Examples of cases where there is a need for repeated overwriting of the same data include synchronization marks, addresses in headers, and where less than a minimum recordable block (or record unit) has been changed. These cases occur in the DVD format, for example, because it uses a set of synchronization marks that always occur at the same place in the data and although the minimum recordable block is 32K bytes, the host computer may operate in 2K byte sectors. A typical operation may change some sectors of a block, while the remainder are rewritten exactly as before.
In some rewritable formats, this overwrite degradation problem is overcome by varying the "start of the writing position" over a number of bytes. This means the same data are never written in exactly the same location. This solution requires the inclusion in the format of a buffer zone or edit gap. It also involves some loss of capacity and makes a rewritable format incompatible with read only formats. In the case of these formats with buffer zones it is desirable to reduce the size of the buffer zones.
Other formats, such as ROM formats, do not contain edit gaps. It is desirable to be able to write to such formats in order to avoid incompatibilities between formats. The disk written with a "ROM" format would be readable by ROM drives that did not know of edit gaps. To write to such ROM formats requires very accurate overwriting of data blocks such that they begin and end at exactly the same location as the previous data. There is therefore no opportunity to vary the location of data to be overwritten relative to preexisting data. However, it is still desirable to overwrite without causing degradation of the phase change medium.